The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
During combustion in a diesel engine, an air/fuel mixture is delivered through an intake valve to cylinders and is compressed and combusted therein. After combustion, the pistons force the exhaust gas in the cylinders into an exhaust system. The exhaust gas may contain particulate matter, oxides of nitrogen (NOx), and carbon monoxide (CO), and the emission of these constituents is regulated for environmental reasons. Thus, vehicles equipped with compression-ignition engines often include after-treatment components for converting, reducing and/or removing particulate matter and other regulated constituents from their exhaust streams. Exhaust gas treatment systems may employ catalysts in one or more components configured for accomplishing an after-treatment process such as reducing NOx to produce more tolerable exhaust constituents of nitrogen (N2) and water (H2O). Reductant may be added to the exhaust gas upstream from an after-treatment component to aid in reduction of the NOx. A Diesel Particulate Filter (DPF) may be employed to capture soot, and that soot may be periodically incinerated during regeneration cycles.
Particulate filters, and other after-treatment components can be effective, but can also increase back pressure as they collect particulate matter, which may include ash and unburned carbon particles generally referred to as soot. As this carbon-based particulate matter accumulates in the after-treatment components, it can increase back pressure in the exhaust system. Engines that have large rates of particulate mass emission can develop excessive back pressure levels in a relatively short period of time, decreasing engine efficiency and power producing capacity. Therefore, it is desired to have particulate filtration systems that minimize back-pressure while effectively capturing particulate matter in the exhaust.
To accomplish both of these competing goals, after-treatment components must be regularly monitored and maintained either by replacing components or by removing the accumulated soot. Cleaning the accumulated soot from an after-treatment component can be achieved via oxidation to CO2 (i.e., burning-off) and is known in the art as regeneration. To avoid service interruptions, regeneration is generally preferred over replacement of after-treatment components. A continuously regenerating trap (CRT) is an after-treatment component that traps particles in the exhaust stream and also includes a catalyst to aid in regeneration.
One way that regeneration may be facilitated is by increasing the temperatures of the filter material and/or the collected particulate matter to levels above the combustion temperature of the particulate matter. Elevating the temperature facilitates consumption of the soot by allowing the excess oxygen in the exhaust gas to oxidize the particulate matter. The regeneration process can be either passive or active. In passive systems, regeneration occurs whenever heat (e.g., carried by the exhaust gasses) and soot (e.g., trapped in the after-treatment components) are sufficient to facilitate oxidation. In active systems, regeneration is induced at desired times by introducing heat from an outside source (e.g., an electrical heater, a fuel burner, a microwave heater, and/or from the engine itself, such as with a late in-cylinder injection or injection of fuel directly into the exhaust stream). Active regeneration can be initiated during various vehicle operations and exhaust conditions. Among these favorable operating conditions are stationary vehicle operations such as when the vehicle is at rest, for example, during a refueling stop.
Some diesel engine systems use cylinder injectors to control temperatures in after-treatment components by adding excess fuel in the cylinder with the intention that the additional fuel be available for increasing temperatures in the after-treatment component. Other diesel engine systems are equipped with after-treatment fuel injector(s), also known as a Hydrocarbon Injector (HCI), to support DPF regeneration by adding fuel directly to the engine exhaust system. Typically, the HCI is used only during DPF regeneration and is commanded on by the engine control system and injects fuel directly into the engine's exhaust gases downstream of the engine's turbocharger, if so equipped. The HCI supplies a measured quantity of fuel into the exhaust gas only during enabled regeneration events. An oxidation catalyst (DOC) converts this added fuel into the heat that's needed to regenerate the DPF by incinerating accumulated soot. DOC temperatures are monitored during regeneration by exhaust gas temperature sensors.
Engine control systems can be used to predict not only when it may be advantageous to actively facilitate a regeneration event, but also to effectuate control over the regeneration process. To exercise active control over a regeneration event, an engine control system often seeks to achieve a desirable temperature in the after-treatment component, or on the DOC, that is conducive to the regeneration process. To accomplish stable control, an engine control module may rely upon a feedback controller such as a proportional-integral-derivative (PID) controller, which calculates an error value as a difference between a measured process variable, such as catalyst temperature, and a desired setpoint. The controller adjusts one or more process variables, such as HCI quantity, seeking to minimize the value of the error until it is within an acceptable tolerance.
Unfortunately, substantial tuning may be required in order to enable a PID controller to provide a stable, responsive control system. For example, if the gains in a PID-based after-treatment regeneration controller are not set properly, HCI may be increased too rapidly, leading to overshoot. If the controller were to repeatedly make excessively large changes to HCI demanded, the desired catalyst temperature may be overshot such that the catalyst temperature oscillates around the desired temperature rather than approaching it. If the oscillations increase over time, then the system is unstable. If the oscillations steadily decrease in magnitude, then the control may be sufficiently stable so long as equilibrium can eventually be achieved. In the case of PID-based after-treatment regeneration controllers, the amount of calibration work required for tuning to achieve a sufficiently stable and responsive control system can be cumbersome and excessive, often requiring use of numerous maps and correction curves, each requiring detail on the order of hundreds of calibration data points in order to produce suitable control stability and response.
Accordingly, it would be desirable to have an improved system and method for controlling regeneration in an after-treatment system.